## Archive for the 'Time' Category

### No Time to Lose

An upcoming symposium, Time for Everyone

“Time for Everyone” is a unique opportunity to learn about the origins, evolution, and future of public time from some of the foremost authorities in many branches of time measurement. From its natural cycles in astronomy, to its biological evolution, to how the brain processes it differently at various stages of life and under different circumstances, to how we find it, how we measure it, and how we keep it, this symposium will explore many facets of this fascinating subject of unfathomable depth. The program has been designed for a diverse audience and the speakers carefully chosen not only for their knowledge, but also for their ability to bring their subjects to life.

Not surprisingly, I’ve met a number of the speakers and heard a few of them give talks (or parts of talks). That list includes Sean Carroll (Arrow of time), Tom Van Baak (amateur “time nut” who did the gravitational time dilation experiment I mention at the end of this post), Geoff Chester (Public Affairs Officer here at the Observatory), and Bill Phillips (Nobel Prize in ’97 for laser cooling and trapping) who is giving the keynote at the banquet.

It’s in the next fiscal year, so the probability of getting to go is not identically zero.

### We Did a Science!

And by “we” I really mean the first author (Steve) who did all legwork of analyzing the copious clock data we generate, and had realized that our continuously-running clocks had an advantage over other groups who have been doing these measurements over longer intervals. I helped out a bit with the clock-building (and clock building-building) and thus data generation, and some feedback.

The arXiv version of “Tests of LPI Using Continuously Running Atomic Clocks” was posted (some time ago, sorry this is late) so you can follow along with the home version of the game, if you wish. Keep in mind that I am an atomic physicist, Jim, and not someone who really works with general relativity past the point of including gravitational time dilation in discussions about timekeeping.

One of the tests of general relativity, or specifically of the Einstein equivalence principle, is that of local position invariance. That is, local physics measurements not involving gravity must not depend on one’s location in space-time. Put another way, there shouldn’t be any effects other than gravitational ones if you do an experiment in multiple locations — the gravitational fractional frequency shift should only depend on the gravitational potential: $\frac{\Delta f}{f} = \frac{\Phi^2}{c^2}$

So you look for a variation in this. One possibility of investigation is to compare co-located clocks of different types as the move to a new location, that could behave differently if LPI were violated. This can arise if the electromagnetic coupling, i.e. the fine structure constant, weren’t the same everywhere. Then clocks using different atoms would deviate from the predicted behavior. Since we’re looking at transitions involving the hyperfine splitting, nuclear structure is involved, so the other possibilities that can be tested are variations in the electron/proton mass ratio and the the ratio of the light quark mass to the quantum chromodynamics length scale. One need not do any kind of (literal) heavy lifting of moving the clocks into different gravitational potentials because the earth does it for us by having an elliptical orbit — we sample different gravitational potentials of the sun over the course of the year.

In order to get the statistics necessary to put good limits on the deviation, other groups have done measurements over the span of several years, but this was because their devices were primary frequency standards, which (as I’ve pointed out before, probably ad nauseum) don’t run all the time, so you only get a handful of data points each year. Continuously running clocks, on the other hand, allow you to do a good measurement in significantly less time. You want to sample the entire orbit along with some overlap — about 1.5 years does it (as opposed to a few measurements per year, where you really need several years’ worth of data to try and detect a sinusoidal variation).

Another key is having a boatload of clocks. Having a selection is especially important for Hydrogen masers, since they have a nasty habit of drifting, and sometimes the drift changes. Having several from which to choose allows one to pick ones that were well-behaved over the course of the experiment. Having lots of Cesium clocks, which are individually not as good (but don’t misbehave as often), allows one to average them together to get good statistics. Finally, having four Rubidium fountains, which are better than masers in the long-term, adds in another precise measurement.

All of the clocks are continually measured against a common reference, so you can compare any pair of clocks by subtracting out the common reference, so we have relative frequency information about all the clocks. The basic analysis was to take the clock frequency measurements and remove any linear drift that was present in the frequency, and check the result for an annually-varying term. The result isn’t zero, because there’s always noise and some of that noise will have a period of a year, but the result is small with regard to the overall measurement error such that it’s consistent with zero (and certainly does not exclude zero in a statistically significant way).

We’ve pushed the limit of where any new physics might pop up just a little further down the experimental road — relativity continues to work well as a description of nature.

### Why Don’t These Things Cost \$50k?

Pop quiz, hotshot: Your really long optical fiber isn’t letting (much) light through, so there’s obviously a break in it somewhere. You need to fix the fiber. What do you do? What…do…you…do?

Obviously, shooting the hostage is not an option here. The fiber is probably buried underground, so it would be really helpful to know where the break is, to a resolution of at least the location of the nearest manhole, so you can go in, find the fault and splice the fiber. The solution is an optical time-domain reflectometer (OTDR). You send a pulse of light down the fiber and measure the delay of any reflection, because breaks (and other faults) tend to reflect the light, as any change in index of refraction causes a reflection. Since the speed of light transmission in a medium is simply c/n, if you can measure the return time of the pulse you can figure out how far way the fault is.

To do this in a helpful way, though, one needs to locate the fault to within a few meters, and light in a fiber will be traveling at around 200,000 km/sec, or 5 nanoseconds per meter, which means we need timing at a level at around the 10 nanosecond level. That sounds like the precision realm of commercial atomic clocks, and that sounds expensive — that kind of clock can run you several tens of thousands of dollars. But there’s an important distinction: an atomic clock gives precision long-term timing, and we don’t need that. If our optical fiber is 100 km long, a round-trip signal will take no longer than a millisecond. In other words, we don’t need a clock that will add fewer than 10 nanoseconds in a day, we just need one that won’t add more than 10 nanoseconds in a millisecond. There is almost 8 orders of magnitude difference in performance in those two systems. Put another way, we don’t want to measure the time, we want to measure a short time interval. A timing error of 10 nanoseconds in a millisecond is 10 parts-per-million, a performance that is easily reached by a cheap quartz oscillator (Here’s a cheap system that does 2 parts per million along with some extra functions we wouldn’t need). As long as the oscillator is calibrated, such a device would be just fine for this task.

Another example of this time interval application is a GPS receiver. These receivers compute your location based on the time difference between signals from multiple satellites, but since the satellites have precise clocks on them and broadcast that information, the receiver only has to measure the difference in those time tags. GPS satellites orbit at altitudes of around 20,000 km, but it’s the differences in the distances that are important to us. Overhead satellites are closest, while ones nearer the horizon are farther away, by a few thousand km. That’s a factor of ~10 greater distance than our OTDR signal (though our speed is very close to c), and we want somewhat better timing, so that puts our needs closer to 0.1 ppm, but this is also achievable, though undoubtedly a little more expensive. The great part about GPS receivers, though, is that you can actually use the timing signals to synchronize a local clock, and gain the benefit of the atomic time on the satellites, which is synchronized to the earth’s atomic time, UTC. (You might recall that such synchronization was initially — and incorrectly — blamed for timing errors in the superluminal neutrino story a little over a year ago. It’s actually quite good.)

### The Tell-Tale Strontium Heart

Beating heart of a quantum time machine exposed

A little vacuum system porn for you.

The lasers are fired through three of the glass shafts emanating from the cube, but must be carefully directed out of the other side to prevent them scattering within the clock, which is why there are six shafts in total.

However:

… the beating heart of a time machine! Or “clock”, as most people call them …

… or possibly “frequency standard” as I like to pedantically point out. Though this being an ion clock, it can probably run for extended periods of time, and one might actually be able to say it’s running as a clock.

I also find the description of the six arms to be curious; normally, trapping schemes send light in both directions. It’s true you don’t want the light scattered in the chamber, but the description implies there are only three, and none of the NPL write-ups I have read say anything about a novel cooling geometry requiring only three beams.

Aaand it gives the Sr transition frequency as an exact number. There should be an uncertainty, since it’s the Cs hyperfine transition which is defined.

So read it for the picture, and not so much the article.

### Merry New Year!

ThankyouforcorrectingmyEnglishwhichstinks

### Get Used to Disappointment

Alan Alda asks scientists to explain: What’s time?

The actor known for portraying Capt. Benjamin Franklin “Hawkeye” Pierce on the TV show “MASH” and more recent guest shots on NBC’s “30 Rock” is also a visiting professor at New York’s Stony Brook University school of journalism and a founder of the school’s Center for Communicating Science.

The center is sponsoring an international contest for scientists asking them to explain in terms a sixth-grader could understand: “What is time?”

This is the followup to last years so-called “flame challenge”, in which he solicited explanations about what a flame is. But there’s a problem: in asking “what is a flame?” the real question is about what is going on in the process of combustion — it’s an analysis of a physical process, and people were asked to explain that. The winner did an excellent job, though Feynman’s pretty good, too.

However, asking “What is time?” is a different beast. I’m guessing they won’t be satisfied with the stock answers of “time is what is measured by a clock” or “time is what keeps everything from happening at once”. However, unlike fire, time isn’t a process that can be broken down into simpler parts, at least as far as we currently know — it’s much more fundamental than that. (It might be an emergent phenomenon, but we haven’t sussed that out to the point where anyone can offer anything as a reasonable answer.) Which puts the question squarely in the realm of philosophy — metaphysics — rather than science.

As I see it, the problem is similar to this: Take a word and try and define it, using only words that are already defined. You can’t. For each word you use in a definition, you need to define that word, and in each definition, you need to define all those words. You end up with circular definitions, so you have to rely on a collection of words that we simply accept because we inherently know what they mean or we give examples rather than a definition. (This is vaguely reminiscent of Gödel’s Incompleteness Theorem — that within a mathematical theory there will be certain arithmetic truths which cannot be proven. Perhaps there is a formal analogue for languages, which would be beyond my experience.) We have some concepts in physics which are fundamental, and it limits what we can do, explanation-wise. We can describe how time behaves and how we can measure it, and use it as a basis of explaining other things, but not what time is.

There is another answer, though it’s still consistent with the thread’s title. Time is a bookkeeping convenience, like other concepts we have (such as momentum and energy). We notice that it has a certain predictable behavior and that it’s useful, so we exploit those properties. In this case, that events happen in a certain order. It matters, for instance, if a piano drops out of the sky and onto a location where you have been standing, if you are there (or somewhere else) when the piano hits. You can be where the piano hit, you can be at home, you can be at the store, you can be at work or school, but all of those are not simultaneously true — there is some orthogonal coordinate that can keep those separate and helps us keep track of what’s going on. Meaning that time helps us solve kinematics problems and other problems in physics.

This is not an argument that time is illusory — it’s real, as far as I’m concerned, but it’s conceptual rather than physical. Which puts it in the same category as momentum and energy and even length. Funny thing, though, is people generally don’t as the same kind of deep question, “What is length?” They can see it, rather than have some other perception, and that seems to be enough, just like the foundational words that make up a language that can’t truly be defined.

Maybe I’m wrong. Perhaps someone out there will rise to the challenge and really be able to explain what time is. But if they can’t, I won’t be disappointed.

### Erased … From Existence!

Dubbed Trace of Time, this eraser-equipped timepiece is constructed primarily from glass and wipes away Dry Erase tasks as those deadlines slip away.

### Writing Press-Release Checks Your Physics Can’t Cash

A Clock that Will Last Forever

Imagine a clock that will keep perfect time forever, even after the heat-death of the universe. This is the “wow” factor behind a device known as a “space-time crystal,” a four-dimensional crystal that has periodic structure in time as well as space.

Bold prediction.

Let me say at the outset that I don’t implicitly trust any press release, especially one that gets quantum entanglement wrong or explains it way too vaguely (“an action on one particle impacts another particle” No!), as this one does, so it’s possible this wasn’t fully vetted by the scientists involved.

But there are other reasons to think they are overselling the experiment here. Let me say at the outset that I find the proposal intriguing; it’s not the physics that is in question, and the claims in the press release are not present in the paper. It’s those promises, of what we’ll be able to do with the experiment, that give me pause. Namely:

Imagine a clock that will keep perfect time forever, even after the heat-death of the universe.

Ok, yeah, about that. It might be fair to claim an atom, or possibly a molecule, will survive the heat death of the universe, but a macroscopic device? The device forms a quantum-mechanical oscillator with an ion trap, requiring a certain configuration of electric and magnetic fields, i.e. this space-time crystal is not a physical crystal. Somehow I doubt that the equipment running it will last forever.

If we lose the expectation that this will last a super long time, we still have the idea that it will be a perfect clock, right? Why is this supposed to be perfect?

The persistent rotation of trapped ions produces temporal order, leading to the formation of a space-time crystal at the lowest quantum energy state.

Because the space-time crystal is already at its lowest quantum energy state, its temporal order – or timekeeping – will theoretically persist [for a long time]

It’s true that a quantum mechanical ground state can persist without violating any laws of thermodynamics, and the ground state of a system has a frequency that is infinitely narrow — excited states have a width that is dictated by the uncertainty relation $\Delta{E}\Delta{t}>\hbar/2$    but a ground state has an infinite lifetime. Thus, no time uncertainty.

However… (you knew this was coming)

The paper shows that the rotation frequency of the ions in the crystal depends on the magnetic field you apply to it. That magnetic field will not have a perfectly precise value — it will have fluctuations in it, which means that the oscillation frequency is not going to be a delta function — there will be uncertainty.

Not only that, but how do you count the oscillations and discern the phase? That introduces error into any clock — the perturbation of measurement. In most atomic clocks you have a transition at some frequency, and the excited state does have some width to it, which is why long-lived transitions are used whenever possible — it means the transition will be narrow — but the proposal for this clock is to measure where a particular ion is by shining a spatially narrow laser on it. So they aren’t leveraging the infinitely narrow state; I don’t think they can. The mental picture I have is that it would be like counting a wheel’s rotation by painting a spot on its rim and counting how many rotations you have. The problem is that any ion is going to have an inherent location uncertainty, and the laser will add to that because the spot will likewise have a spatial extent. So even if that’s small, it won’t vanish — there will be a measurement uncertainty introduced, on top of the frequency uncertainty from the magnetic field. Not perfect.

Go ahead and blame me for being the reason we can’t have nice things that are perfect and last beyond the heat death of the universe.

### The March of the Metro Gnomes

Yay for mechanical coupling, which is enough of an effect to drive these into synch as long as they are all naturally oscillating close to the same frequency. This same effect is/was used in clock shops — pendulum clocks hung on a wall would similarly synchronize, giving the illusion that they must all be wonderfully precise clocks, to all be ticking at the same rate and in phase like that.

Spoiler alert: nothing dramatic happens in the last minute of the video — they just tick away. It’s tempting to try a cadence (There she was, just a-walkin’ down the street…), but the ticks are a bit fast.

### Nature, Dissin’ the Maser

Microwave laser fulfills 60 years of promise

Because of this [low power] impediment, most in the field gave up on masers and moved on to lasers, which use the same principles of physics, but work with optical light instead of microwaves. Lasers are now used in applications ranging from eye surgery to CD players.

The poor maser lived on in obscurity. It found only a few niche uses, such as boosting radio signals from distant spacecraft — including NASA’s Curiosity Mars rover. Those masers work only when cooled to less than ten degrees above absolute zero, and even then they are not nearly as powerful as lasers.

To paraphrase Ray “Bones” Barboni, this is the exact frikkin’ thing I needed. A little pique after a blogcation to get the blood going again. And to quote Jules Winnfield, “Well, allow me to retort.”

First of all, “microwave laser” is just … wrong. The maser came first, so popularity aside, you don’t just ignore the history. That’s like touting a cover song while ignoring the songwriter who first recorded it. Blasphemy.

Second, and more importantly, the “first practical maser”? The mind boggles. Well, my mind does, anyway. Hydrogen masers have been the best atomic clocks at time scales out to a day or so for quite a while, and even with the advent of laser-cooled atomic clocks in the past decade, they only surpass masers after about a day of integration. (This is why the even more advanced optical clocks you read about every few months cannot be called better, in some sense — they don’t yet run long enough to make a significant contribution to timekeeping). You can make the argument that the world’s timekeeping, backbone for GPS and other timing-dependent technologies is living in obscurity, but I can’t see how that isn’t practical.

### A Stopped Clock That’s Right Four Times a Year

A sundial that spells out the equinoxes and solstices.

### Move Along at c; Nothing to See Here

MINOS reports new measurement of neutrino velocity

[T]he new MINOS study significantly reduces the systematic errors of its earlier work with detailed measurements of the behavior of the experiment’s GPS timing system, improved understanding of the delays of electronic components at every stage of the MINOS detectors and the use of upgraded timing equipment, designed and implemented with the assistance of the National Institute of Science and Technology and the United States Naval Observatory.

Applying these improved understandings, the MINOS collaboration measures a neutrino arrival time for travel between Fermilab and Soudan, Minn., that is consistent with the expected travel time at the speed of light. The difference between the measured and calculated times is -15 ± 31 nanoseconds, indicating no observable effect.

Gotta include the plug for the home team.